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About Notifiable Infectious Diseases and Conditions Data

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Introduction

Public health surveillance of national notifiable infectious diseases and conditions helps public health authorities monitor the effect of these diseases and conditions, measure the disease and condition trends, assess the effectiveness of control and prevention measures, identify populations or geographic areas at high risk, allocate resources appropriately, formulate prevention strategies, and develop public health policies.

Provisional data on the reported occurrence of national notifiable infectious diseases and conditions are published weekly, except for data about tuberculosis, which is published quarterly. After each reporting year, staff in state and territorial health departments finalize reports of cases for that year with local or county health departments and reconcile the data with reports previously sent to CDC throughout the year.

These data are compiled in final form and are published annually, to represent the official and archival count of cases for each disease and condition. The data published annually are approved by the appropriate chief epidemiologist from each submitting state or territory before being published as a final annual statistics. Data published elsewhere might differ from NNDSS data cited here because of differences in the timing of reports, the source of the data, or surveillance methodology.

Completeness and Surveillance System Characteristics

The completeness of information on notifiable diseases and conditions is highly variable and related to the disease or condition being reported. Incidence data are presented by the Morbidity and Mortality Weekly Report (MMWR) week and year (https://wwwn.cdc.gov/nndss/document/MMWR _Week_overview.pdf) assigned by the state or territorial health department, with some exceptions, including human immunodeficiency virus (HIV) (presented by date of diagnosis), tuberculosis (presented by date that the reporting jurisdiction verified that the case met the criteria in the national surveillance case definition), domestic arboviral diseases (presented by date of illness onset), and varicella deaths (presented by date of death). The calendar days corresponding to MMWR weeks for MMWR years are available on this site: https://wwwn.cdc.gov/nndss/downloads.html. MMWR month is derived from MMWR weeks. Data were reported by the jurisdiction of the person’s “usual residence” at the time of disease or condition onset (https://wwwn.cdc.gov/nndss/document/11-SI-04.pdf). For certain national notifiable infectious diseases and conditions, surveillance data are reported independently to various CDC programs. For this reason, surveillance data reported by other CDC programs might vary from data reported here because of differences in: 1) the date used to aggregate data (e.g., date of report or date of disease or condition occurrence), 2) the timing of reports, 3) the source of the data, 4) surveillance case definitions, 5) publication criteria, and 6) policies regarding case jurisdiction (i.e., which jurisdiction should submit the case notification to CDC). In addition, the “date of disease occurrence” of conditions might vary. For infectious diseases, the meaning of the “date of disease occurrence” varies across jurisdictions and by disease and might be a date of symptom or disease onset, diagnosis, laboratory result, reporting of a case to a jurisdiction, or notification of a case to CDC.

Using and Interpreting These Data

These data are useful for analyzing infectious disease or condition trends and determining relative infectious disease or condition numbers. However, reporting practices affect how these data should be interpreted. Infectious disease and condition reporting is likely incomplete, and completeness might vary depending on the infectious disease or condition and reporting state. The degree of completeness of data reporting also might be influenced by the diagnostic facilities available, control measures in effect, public awareness of a specific infectious disease or condition, and the resources and priorities of state and local officials responsible for public health surveillance and for controlling infectious diseases and conditions. Finally, factors such as changes in methods for public health surveillance, introduction of new diagnostic tests, or discovery of new infectious disease or condition entities can cause changes in reporting that are independent of the actual incidence of infectious disease or condition.

Public health surveillance data are published for selected racial and ethnic populations because these characteristics can be risk markers for certain notifiable infectious diseases or conditions. Race and ethnicity data also can be used to highlight populations for focused prevention programs. However, caution must be used when drawing conclusions from reported race and ethnicity. Different racial and ethnic populations might have different patterns of access to health care, potentially resulting in data that are not representative of actual infectious disease or condition incidence among specific population groups. In addition, not all race and ethnicity data are collected or reported uniformly for all infectious diseases and conditions; for example, the recommended standard for classifying a person’s race or ethnicity is based on self-report. However, this procedure might not always be followed.

Surveillance data reported to NNDSS are in either individual case-specific form or summary form (i.e., aggregated data for a group of cases). Summary data often lack demographic information (e.g., race); therefore, the demographic-specific rates presented in this summary might be underestimated.